The invention relates to an optical fire detector in which a radiation-emitting means is arranged to emit a beam of radiation and a radiation detecting means is arranged to receive the beam of radiation after it has passed through an intermediate air medium and to provide an alarm upon an obscuration of the beam of radiation.
In the Swedish Pat. No. 335,080 an optical fire detector is described in which a reduced sensitivity to disturbances is obtained by dividing a beam of radiation into two parallel beams of radiation which pass separated measuring chambers in order to be influenced differently by fire and the intensities of which are measured separately and are compared mutually, an alarm being provided upon a predetermined difference between the intensities.
The British Pat. No. 1,405,615 describes an optical fire detector which can use an arbitrary measuring chamber, for example, a chamber arranged so that a beam of radiation passes across the chamber. Here a reduced sensitivity to disturbances is obtained by using a source of radiation which is intensity modulated with a definite frequency and a radiation detector which is frequency selective with respect to the frequency of the intensity modulation of the radiation source. This type of optical fire detector is suitable to respond to heat as well as to smoke. Its ability to provide an early fire alarm is, however, quite limited due to the fact that there is no difference measurement of the intensities of two beams of radiation. In addition, the above-mentioned fire detector described in the Swedish Pat. No. 335,080 is unable to provide an early fire alarm since it takes considerable time to carry out a sample of the air from a secured chamber to a separate measuring chamber.
An object of the invention is of providing an optical fire detector which can use an arbitrary measuring chamber and which combines a low sensitivity to disturbances with the capability to provide an early fire alarm.